Add unit test to os-agent API

This patch adds unit tests for os-agent API to test updating
and deleting a agent with not existing agent id.

Change-Id: Id266b0449deff6bd3715cbd2b5403031a2e34eeb
Fixes AttributeError with api sample test fail

Avoid AttributeError which could be raised with api sample tests
when a string is expected but a non string type is returned
instead. This makes debugging api sample test failures easier as
the expected and actual values are displayed in the error message
rather than an AttributeError exception.

Change-Id: I4d124e4d80779c4431926619f0ca876a513a8462
Merge "Add unit test to os-agent API"
Merge "Fixes AttributeError with api sample test fail"